A gear mechanism is known which includes a sun gear, a planetary gear, and an arm configured to swing around the rotary shaft of the sun gear together with the planetary gear. Further, an image forming apparatus in which the above described gear mechanism is placed is known.
In such a gear mechanism, if the swing load of the arm is too light, the arm may be moved undesirably due to gravity or oscillation, or if the swing load of the arm is too heavy, the arm may not swing as desired. In the above described cases, the operation may become unstable. If the arm does not operate as desired, the apparatus including the gear mechanism with arms may not operate as desired.